linux-kernel - Re: [PATCH 0/3] make vm_committed_as_batch aware of vm overcommit policy

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Message-ID: <20200529160401.GJ93879@shbuild999.sh.intel.com>
Date:   Sat, 30 May 2020 00:04:01 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Andi Kleen <andi.kleen@...el.com>
Cc:     Qian Cai <cai@....pw>, Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Kees Cook <keescook@...omium.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        "Chen, Tim C" <tim.c.chen@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] make vm_committed_as_batch aware of vm overcommit
 policy

On Fri, May 29, 2020 at 08:50:25AM -0700, Andi Kleen wrote:
> >  
> >  	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> > -	if (ret == 0 && write)
> > +	if (ret == 0 && write) {
> > +		if (sysctl_overcommit_memory == OVERCOMMIT_NEVER)
> > +			schedule_on_each_cpu(sync_overcommit_as);
> 
> The schedule is not atomic.
> There's still a race window here over all the CPUs where the WARN_ON could
> happen because you change the global first.
 
The re-computing of batch number comes after this sync, so at this
point the batch is still the bigger one, and won't trigger the warning.

> Probably you would need another global that says "i'm currently changing
> the mode" and then skip the WARN_ON in that window. Maybe a sequence lock.
> 
> Seems all overkill to me. Better to kill the warning.

Yes, the cost is high, schedule_on_each_cpu is labeled as "very slow"
in the code comments.

Thanks,
Feng

> 
> -Andi